Ok I was once hit by a worm virus when in college in the late 90s, that attacked all the PCs of the network through an unprotected port. At that time we didn't use firewalls. It didn't do much damage, just forced the PC to shutdown
So although I cannot state it in axiomatic form, judging from my experience, an intrusive anti-virus program isn't necessary for a healthy computer. So what's the point of anti-virus programs? They do a pretty good job at slowing down your computer to a halt, mistrusting and checking everything, what you open, where you browse, throwing up all sorts of imaginary false positives. Whenever I get some really weird xplorer² support "problem" it invariably turns out to be the doing of a paranoid overjealous AV program. So they serve no purpose, pointlessly wear down your computer's resources, and give you needless worry and inconvenience. It's like you prance around in a heavy silver armour every day, just in case! And in the end of the day, no antivirus would have saved you from the major 0-day malware offensives. Just uninstall the darn thing and reclaim your computer sanity.
How do viruses get on your system anyway? In one way or another a malicous program must execute on your PC. This could be a dodgy program you installed, or an email attachment you got tricked into opening, or if you are really an old timer, from an infected floppy disk! There are also these "drive by" infections when you browse an infected website that e.g. exploits an unpatched flash player vulnerability. But in the end of the day a program must execute, one way or another. The maxim that keeps you safe is:
One last point is about ransomware, you know the type of virus that locks your documents and then asks money to give your stuff back. That is really stuff that nightmares should be made of. It is also possible to happen even if you are running under a restricted account because a virus doesn't need to install in %WINDIR%; it can run as nicely off the temp directory and has full access to your personal documents. Your OS is protected but your documents are not. If you are unlucky and pick one of these nasties, the solution is simple: use regular backups.
PS. Many times I read scary articles about how vulnerable is internet explorer or the adobe flash player, and the immense risk to life, limb and machine, for people that are foolish enough to still use them (the rumors are usually fuelled and exaggerated by antivirus companies, I wonder why?). Again speaking of personal experience, doing all my browsing with IE for ages, I can assure you that there is no case of infection. And if there is a 0-day vulnerability waiting to hit upon us, the worse that can happen is along the lines of drive-by ransomware. And most probably you must have been lured into visiting some funny website, so what were you doing there in the first place? :P